A typical Fabry-Perot cavity optical modulator has two high and equal reflectivity dielectric mirrors separated by a gap. The mirrors have a nominal thickness equal to one-quarter of the operating wavelength ("quarter-wave") of the modulator. Usually, one of the mirrors is fixed and the other is movable towards the fixed mirror. As a voltage is applied across the two mirrors, the movable mirror moves toward the fixed mirror, decreasing the gap between the mirrors. Modulator reflectivity varies with the size of the gap. See, for example, Marxer et al., "MHz Opto-Mechanical Modulator," Transducers '95--Eurosensors IX, Royal Swedish Academy of Eng. Sciences, Stockholm (June 1995), pp. 289-292.
Fabry-Perot cavity modulators are high finesse devices. Finesse is given by the expression: F=4R/(1-R).sup.2, where R is the mirror reflectivity, which is the same for both mirrors. Mirror reflectivity is given by (n.sup.2 -n.sub.o)/(n.sup.2 +n.sub.o)!.sup.2, where n is the refractive index of the material comprising the mirror and n.sub.o is the refractive index of the underlying medium, typically air. Thus, for a Fabry-Perot cavity modulator having polysilicon mirrors (n=3.5, n.sub.o =1): R=72 percent and F.apprxeq.37. Such modulators can achieve high reflectivity, i.e., low insertion loss; with theoretical device reflectivity exceeding ninety percent. Unfortunately, as a result of its high finesse, a Fabry-Perot cavity modulator typically has a relatively narrow operating bandwidth, e.g., about 5 nanometers (nm) centered around a center operating wavelength.
A lower finesse modulator is disclosed in U.S. Pat. No. 5,500,761. That modulator has a quarter-wave movable membrane separated by a gap from a substrate layer. The movable membrane incorporates a layer having a refractive index that is about equal to the square root of the substrate's refractive index. The movable membrane and the substrate have equal reflectivities. Under applied voltage, the movable membrane moves toward the substrate layer, changing the size of the gap. Changing the gap size results in a change in device reflectivity.
As previously noted, the modulator described in U.S. Pat. No. 5,500,761 has a lower finesse than a typical Fabry-Perot cavity device. In particular, membrane and substrate reflectivity R is about 31 percent and finesse is about 2.6. Such a modulator is characterized by a maximum device reflectivity of about seventy-two percent. Due to its lower finesse, that modulator advantageously possesses a significantly broader operating bandwidth than a Fabry-Perot modulator, in particular about 50 to 60 nm around a center operating wavelength.
Thus, in the prior art, one can select a modulator providing low insertion loss but narrow operating bandwidth, or higher insertion loss and broader operating bandwidth. It would be desirable, however, to have an optical modulator providing the best characteristics of the prior art modulators. Such a modulator would have the low insertion loss of the standard Fabry-Perot cavity modulator, and also possess the relatively broad operating bandwidth of the modulator disclosed in U.S. Pat. No. 5,500,761.